This invention relates to an improved process for recovering tin or other non-ferrous metal values from low-grade ores, concentrates, or mineral mixes which consist mainly of iron sulphide minerals.
Conventional tin smelting processes are applied to essentially sulphide-free, high grade concentrates such as those derived from alluvial sources, containing tin in excess of 60%. These concentrates, which are prepared by conventional mineral dressing methods are treated in a two stage reverberatory furnace operation. The concentrates are charged to the reverberatory furnace at 1200.degree. C. with fluxing materials to cause the charge to melt and form a fluid slag which is reduced by a suitable reductant such as coal to produce crude tin. The tin product from the first stage is sufficiently low in iron and other impurities to permit refining by conventional methods. The slag which contains typically 5 to 10% tin, must be granulated and treated in the second stage to recover this tin. It is mixed with additional reductant and treated at approx. 1400.degree. C., to produce a discard slag containing 1.5 to 3 percent tin and a metallic product with a high iron content (hard-head) which is recycled to the first stage.
Low grade concentrates from hard-rock sources not only contain larger quantities of gangue minerals (and therefore produce large volumes of slag per unit of crude tin produced) but usually have higher iron-to-tin ratios, which leads to the formation of large quantities of high melting point hard-head with high iron content which cannot be dealt with in the normal two stage recycle. These two factors make it uneconomical to treat low grade concentrates in conventional smelters.
In the treatment of low grade concentrates it is usual to employ processes which involve a slag fuming operation to remove tin from the system either as a fume of stannous oxide SnO (by appropriate control of oxidation/reduction conditions) or as a fume of stannous sulphide SnS (by injection of sulphur or pyrites to control sulphidizing conditions, together with coal or a hydrocarbon fuel to control reduction conditions). Careful control of the charge composition and rates of addition of reagents is required to maintain fuming conditions without matte formation. Any matte which is formed is treated separately to recover its tin values or recycled as a sulphidizing agent. This type of slag-fuming operation is generally employed as a slag cleaning stage for slags derived from the first stage of conventional smelting operations. It allows the iron entering the smelter to be discharged in the discard slag, and thus avoids the buildup and excessive recycle of iron. In all such slag fuming processes the fuel and sulphidizing agent are injected into the slag layer, below its surface, and matte formation is avoided where possible. It is possible, though not usual, to charge concentrates which may or may not contain iron sulphides, to the fuming furnace.
This type of process is represented in the prior art by:
1. U.S. Pat. No. 2,304,197 Dec. 8, 1942, W. H. Osborn PA1 2. D. V. Belyayev, The Metallurgy of Tin, Pergamon Press, Oxford, 1963, p. 88. PA1 1. U.S. Pat. No. 2,600,351--Wells, Thompson & Roberts, Dorr Company 1952 PA1 2. U.S. Pat. No. 1,847,991--Sulman & Picard 1932 PA1 3. Australian Pat. No. 3735/26--Krupp 1926. PA1 (a) the sulphides are consumed as rapidly as they are added, PA1 (b) the sulphur content of the mixture of slag and dispersed matte is not permitted to rise above six percent (above which the matte will form a separate bottom phase).
An alternative approach dispenses with both the preconcentration step and the conventional smelting step; it is basically an ore fuming process which is applicable to ores or blends which consist mainly of iron sulphide minerals with relatively small quantities of other minerals, including cassiterite.
It will be appreciated that the avoidance of the preconcentration stage and the use of a high-temperature fuming process on the whole of the ore as mined introduces some very special economic restraints. For a given production of tin from a 1% tin ore, as distinct from say a 20% concentrate, the total material throughput will be twenty times greater. Thus capital costs must be optimized and the specific throughput of the plant must be as great as possible, to retain economic viability.
Of even greater importance from the economic point of view is the cost of the energy required to heat the whole of the ore to the fuming temperature. In an ideal ore fuming process this heat must be supplied by burning of the components of the ore without the need for additional fuel. These processes are in general therefore only applicable to ores containing sufficient iron sulphides to provide all the process heat from their combustion. (Of course, if the ore is supplemented by concentrates containing a greater proportion of tin, it may become economical to use some supplementary fuel).
In brief, an ore fuming process should be a high intensity process with high specific throughput, and should be autogenous.
Two distinct variants of sulphide ore fuming processes have been described in the prior art. These are:
(1) Non-slagging processes which are applied to iron sulphide ores of the type for which the present invention is intended. In these processes the essential feature is that the temperature is maintained at a sufficiently high level to permit volatilization of tin sulphides to occur, but at a sufficiently low level to avoid incipient melting of the charge. The processes may thus be carried out in fluidized beds, multihearth roasters or shaft furnaces and the residues are free-flowing particulate solids. Such processes are represented in the prior art by the following patents:
(2) Slag forming processes applied to sulphide ores. This group of processes like group 1 is also applied to ores consisting mainly of iron sulphide minerals. In this group the temperature of the charge is allowed to rise sufficiently due to burning of the iron sulphide minerals that melting and slag formation occur and the final residue is an iron silicate slag. Formation of a matte is avoided if possible, but if it does form, it is collected in the hearth, tapped from time to time and treated to recover its tin content. This type of process is represented in the prior art by Trostler and Carlsson, U.S. Pat. No. 2,219,411 (1940) and Australian Pat. No. 109,112 (1939) and by Brovkin et al., Brit. Pat. No. 1,391,572 (1975).
(The present invention falls into this group, and is related to the process of Carlsson and Trostler, in that it is intended for the treatment of ores or concentrates consisting largely of iron sulphides with sufficient silica naturally occurring or deliberately added to cause all the iron produced in the residue from burning of the iron sulphides to form an iron silicate (fayalite) slag. The molten slag is the residue from the process and is tapped for discard.
In the treatment of certain sulphidic tin ores from the West Coast of Tasmania, conventional mineral dressing treatments failed to achieve recoveries better than 30% with a concentrate grade of 30 percent tin. Thus conventional treatment involving preconcentration and smelting (even those smelting processes applicable to low grade concentrates) could not be applied.
Our early attempts towards development of an autogenous ore fuming process for treatment of these pyritic tin ores, involved the non-slagging approach, i.e. heating the ore in such a way that the tin was sulphidized and volatilized without allowing the temperature to rise to the level where slagging reactions could cause agglomeration and sticking of the charge. In this process the volatilization and recovery of tin is achieved in the following way:
(1) The ore is fed to a shaft or multi-hearth furnace co-currently with hot gases derived from combustion of residues in later stage (3). Co-current operation retains the partial pressure of sulphur released from the pyrite as the temperature of the ore rises.
(2) The gases from the co-current volatilization (stage (1)) containing sulphur, sulphur dioxide, carbonyl sulphide (COS), nitrogen, carbon dioxide and stannous sulphide, are collected and burnt to convert all sulphur and SnS to SO.sub.2 and SnO.sub.2. The gases are cooled and the SnO.sub.2 fume collected.
(3) The heated residue from stage (1) which contains pyrrhotite, silica and minor amounts of other minerals is passed through a suitable lock system into a reactor where it is burned (by counter-current contact in a separate compartment) with air diluted by cooled tail gases from stage (2). The proportion of diluent is adjusted to limit the temperature of the burning mass to below the temperature of incipient fusion, while producing a sufficient volume of combustion gases of sufficiently high temperature to carry out the co-current heating of the ore.
When attempts were made to conduct this process autogenously in a simulated practical reactor system it was found to be impossible to achieve a suitable combination of gas temperature and gas volume from stage (3) without causing slagging reactions and consequent sticking and agglomeration of the reacting mass. Furthermore, even in the absence of these problems the practical difficulties associated with (a) balancing of reaction rates in the two stages, (b) transfer of gases at very high temperatures from stage (3) to stage (1) and pressure balancing between reaction zones, and (c) collecting fume from excessively large volumes of recycled gas, rendered the process unattractive.
The blast furnace pyritic smelting process described by Carlsson and Trostler appeared to offer the only remaining possibility in the prior art for the treatment of these ores. However tests conducted in a small shaft reactor showed that incipient fusion above the melting zone caused bridging of the charge, and the process could not be made to operate. There is no evidence in the literature that this blast furnace process was ever operated commercially.
The problem accordingly remained of finding a process for concentrating cassiterite from the ore which is not subject to the disadvantages of the prior art processes discussed above.